Show 'File saved' not Cell Metadata Updated

[DonJayamanne]

For #207968

Work around for better Notebook Timeline entries
In notebooks when adding a cell, we (built in ipynb serializer extension) performs additional edits as a side effect of adding a cell

• As a result, we do not see the Insert Cell in Timeline as the last edit operations is Update Cell Metadata
• As Cell Metadata Updated is not a user friendly operation we would prefer to replace this label in favour of File Saved (as is done today for Text Buffer edits)

Looking at how this is done for text editor, we need to ensure the code === 'undoredo.textBufferEdit'

@bpasero
Wanted to check if this simple work around is sufficient, or whether we should come up with a different strategy.

We want notebook edit operations to end up in the line return undefined, thats the code path that leads to File Saved in the timeline view.

	private resolveSourceFromUndoRedo(e: IWorkingCopySaveEvent): SaveSource | undefined {
		const lastStackElement = this.undoRedoService.getLastElement(e.workingCopy.resource);
		if (lastStackElement) {
			if (lastStackElement.code === 'undoredo.textBufferEdit') {
				return undefined; // ignore any unspecific stack element that resulted just from typing
			}